During the drilling or completion of oil and gas wells, it is desired to test or evaluate the well's production capacity by isolating the well bore to be tested. Generally, such tests have been performed by logging devices--having semiconductor electronics and probe mechanisms--that are lowered into a well once the drill string has been withdrawn, for either well-completion operations or mid drilling formation surveys. Such tests include formation permeability evaluations made from the pressure change at the well bore formation surface using one or more draw-down pistons. Furthermore, the amount of time, money and resources for retrieving the drill string and running a test rig into the well bore is significant.
An example of a testing system used for well evaluation is provided in U.S. Pat. No. 4,635,717, issued Jan. 13, 1987 to Albert H. Jageler, entitled "Method and Apparatus for Obtaining Selected Samples of Formation Fluids." The testing system disclosed is an inflatable double packer for isolating an interval of the bore hole for removing fluids from the isolated interval. The system is lowered into an uncased bore hole on a conventional wireline after the drilling string has been removed.
But it is highly desirable to conduct early evaluation tests while drilling. That is, without the need to first retrieve the drill string and then make a trip for separate and distinct evaluation apparatus. First, downhole measurements while drilling would allow safer, more efficient, and more economic drilling of both exploration and production wells. Second, being able to evaluate a well repeatedly during the drilling process would allow making earlier development decisions regarding well completion and further tests, and potentially avoiding consumable costs, such as drilling-fluids and drill-bits. Third, tests can be conducted when the formation is freshly penetrated, thus minimizing the likelihood that the tests can be affected by drilling-fluid invasion into the formation. Otherwise, before an uncontaminated sample of connate fluid can be collected, the formation around the well bore that contains forced drilling-fluid filtrates must be "flushed out."
But the detrimental effect of the harsh drilling environment on delicate test equipment has been a strong deterrence for early evaluation systems used in combination with the well drill string. First, drilling string equipment must be capable of withstanding severe subterranean heat and pressure forces compounded by friction, abrasion, and compression, shock, and vibration forces generated along the drill string while rotating and urging a drill-bit into a subterranean formation. Second, a drilling-fluid is circulated under high pressure through the drilling string and back through the annular well bore space surrounding the drill string to cool the drill-bit and to flush formation cuttings to the surface.
Typically, conventional testing devices cannot accommodate high flow rates and a small pressure drop across the tool or variant shock, vibration or torque forces encountered on conventional strings when drilling.
To further complicate the drilling environment, drilling-fluid circulation during well development operations must be maintained because it serves as a first line of defense against a blowout or loss of well control. The circulated drilling-fluid serves to maintain a hydrostatic head or pressure exerted against the well bore surface to contain formation pressure.
Circulating drilling-fluid also helps prevent "stuck pipe," which typically occurs when drilling has stopped for any number of reasons, such as a rig breakdown, or a directional survey or another nondrilling operation. Stuck pipe can occur with the build up of filter cake--a layer of wet mud solids--that form on the surface of the well bore in permeable formations. The hydrostatic pressure of the circulating drilling-fluid can then press the drill string into this filter cake where pressure is lower than the hydrostatic pressure of the drilling mud. That is, the pressure differential between the inner diameter and the outer diameter of the pipe causes the pipe to lodge or stick in the well bore. To limit the chance for stuck pipe, drilling-fluid circulation is maintained to lubricate the pipe string within the well bore, and the pipe is kept moving vertically or rotating.
Conventional wireline test devices are incapable of withstanding the drilling environment. Commonly, wireline devices employ a well bore sealing device, such as a packer, to isolate discrete portions of the well bore to conduct formation testing. First, these sealing devices have expandable elements that cannot endure the frictional forces encountered during drilling, and are typically destroyed by the time they are needed for testing. Second, these sealing devices block the drilling-fluid circulation through the annular space between the drill string and the wall of the well bore, increasing the chances for a well blowout or a stuck pipe string.
Thus, there exists a need for an early evaluation system that can travel with the drilling string for selective deployment and redeployment in the well bore while in the drilling environment.